Mobile communication has made considerable progress in recent years. Starting from a traditional circuit switched (CS) approach as pursued in the Global System of Mobile Communication (GSM), the technological evolution has made its way towards packet switched (PS) systems such as those operating for example on the basis of the Internet Protocol (IP). An example of such communication system is the Universal Mobile Telecommunication Standard (UMTS), or a system known as IMS system (IP Multimedia System). While in former systems, speech or voice was a main aspect of “media” as payload data to be conveyed via the communication system, nowadays all kind of media are focused on as payload, e.g. speech, music (audio), images (still images i.e. pictures as well as moving images i.e. videos), executable program code, text data, and many other types or combinations of media types. “Media” as used in this document is to be understood as not being limited to a specific type of media but is intended to be used in its broadest possible meaning.
With the emerging development of new systems, those systems coexist in parallel and interoperability is one aspect of the development. In particular in case of users (represented by their terminals, e.g. mobile stations MS in GSM or user equipment UE in UMTS and/or IMS) that move and/or roam throughout the communication networks, they expect seamingless services of high quality to be provided to them, irrespective of the current communication system in which they camp.
In Release 8 of the 3rd Generation partnership project, 3GPP Rel-8, a study has started on how to provide IMS (IP Multimedia Subsystem) service via CS (circuit switched) access. This study is covered within 3GPP under the topic “IMS centralized service control”. A corresponding Technical Report TR 23.892 summarizes the results of this study.
IMS service provisioning relies on IP connectivity, which is at least for a VoIP (Voice over IP) data stream not always available, e.g. when roaming in GSM access systems. The above study tries to overcome this limitation by studying how to re-use CS access or how to accomplish access with limited PS capability.
For ICS (IMS Centralized Services) the service control, and as far as possible the service provisioning, shall be in the IMS domain. This leads to the need to have an extra signaling connection between a terminal and the IMS domain, transferred by the CS domain. So, it is envisioned that the session signaling and bearer setup signaling are separated as shown in FIG. 1.
FIG. 1 shows a terminal accessing a radio access network via an illustrated access node thereof, i.e. a Base Station BS or Node_B. Such access is accomplished via either a 2G or 3G radio link. Both 2G and 3G links are assumed to be circuit switched CS and both links are assumed to be packet switched PS capable. From the access node as such, signaling and payload connections extend through the access network towards the IMS domain.
A call from/to a terminal represents a logical association between two endpoints, one of which being the terminal. In a CS domain, the call relies on a bearer connection between the endpoints. In a PS domain, the call relies on a session established between the endpoints. The call comprises signaling and payload (media) delivered, and thus a signaling channel is established as well as a media channel. Each of the channels thus may in principle rely on PS or CS access domain.
Thus, as shown in FIG. 1, a CS based bearer control signaling path extends via the access network to the IMS Domain and also a PS based session control signaling path extends via the access network to the IMS Domain.
Within the IMS domain, so called IMS centralized services (ICS) are provisioned. Entities configured to assist and/or accomplish this are not shown as such in FIG. 1 but are referred to as ICS centralized services control functionality ICCF. A terminal requesting for such services and/or being provisioned with such services is referred to as ICS UE.
It is to be noted that the examples given in conjunction with the present invention are not intended to be limiting in any way. Rather, specific terminology as used in some documents referred to in this document is adopted as an example only to simplify the description of the present invention and to illustrate a particular case to which the present invention may advantageously be applicable. This, however, is not intended to exclude any other field of application for the present invention, as long as the access paths towards a terminal rely on at least two distinct access principles or access domains such as CS and PS.
The above mentioned technical report TR23.892 considers three different network scenarios referred to as scenario A, B and C for centralized IMS service control, which can briefly be described as follows:
Scenario A: The serving access network is an IP-CAN (IP connectivity access network) fully capable of transporting bi-directional speech media or media/content in general. Here both, i.e. media transport and session control signaling is carried over the IP CAN;
Scenario B: The serving access network is a CS access only. Here both, i.e. media transport and session control signaling is carried over a CS domain access;
Scenario C: Both, CS domain access and an IP-CAN are present as serving access networks, but the IP-CAN is not capable of transporting bi-directional speech media. Here, media transport is carried over CS access and session and media control signaling is carried either over the CS domain access or over the IP-CAN which is not capable of transporting bi-directional (speech) media.
In such scenarios, many aspects need to be reconsidered; one of these is the handling of terminating calls towards users, which have service control only in the IMS. These users are called ICS users in this document (illustrated in some Figures as ICS UE).
The IMS is mostly unaware of the PS capabilities of the access network. Thus, as described above in relation to scenarios A, B, and C, the access network might thus be able to transfer both, the session control signaling and the (speech) media via PS domain, or the access network might be able to support session control signaling via PS domain (described as I1-ps below), but the access network might not able to transfer media such as speech via e.g. VoIP and therefore has to use CS domain access for that. Or the access network is even not multiRAB/DTM (multiRAB=multiple Radio Access Bearer, DTM=Dual Transfer Mode) capable, i.e. does not allow PS and CS to exist/be used in parallel. In that case even I1-ps can't be used for session control signaling and session control signaling must then be handled via CS access (described as I1-cs below).
One may imagine that the user experience is quite worse if the IMS selects to use an access network not capable of handling VoIP for a speech connection. Or if the ICS UE is roaming in or to a GSM cell from a DTM/Multi-RAB capable cell, which is thus not DTM capable, and terminating session setup failed for that reason.
The above scenarios are being dealt with in release 7 of 3GPP, 3GPP Rel-7, for Voice Call Continuity VCC (TS 23.206v720) and to some extent described within the Rel-7 Study on Optimizations and Enhancements for Real-time IMS communication (TR 23.818v0.10.0). For example chapter 11 of TR 23.818 analyses domain selection, also for terminated call routing. For domain selection, it can be distinguished between service domain selection SDS on one hand and access domain selection ADS on the other hand.
A call terminating at a terminal UE and routed to the terminal has thus to be analyzed in terms of terminating service domain selection TSDS and whether to use CS termination services or IMS termination services. Also, a call has to be analyzed in terms of terminating access domain selection TADS and whether to use IMS access or CS access.
The function TADS-IMS above selects either IMS access, which includes PS access, or CS access for allocating the bearer channel. For the access domain selection decision, the TADS-IMS takes the following factors into account:
The state of the UE in the circuit switched domain, i.e. whether detached or attached. The state of the UE in the IMS, i.e. whether registered or deregistered. The domain used by an existing session (i.e. whether PS or CS domain). The media components included in the incoming IMS multimedia telephony. User preferences and/or operator policy.
A network operator may configure his operator policies to give preferences to one or the other access domain, but at least in roaming scenarios these preferences may not be suitable and problems may occur as described hereinabove. The access domain selection in the IMS thus selects the access domain “blindly”.
But for ICS (IMS centralized services) the situation is even more difficult, because between the UE and the IMS two channels may be active, one to control the session and another one to transfer the media. VCC Rel-7 and the Study on Optimization and Enhancements for Real-time IMS communication propose a function TADS-IMS which selects the same access network for the session control channel and the media channel.
In above outlined scenarios for terminating calls terminating at roaming terminals various problems may arise which degrade the service experience for a user.